Light Metering Principles

[exmpa]

I have a rather special problem. I coach a blind rifle shooter and part of the special equipment used is an acoustic sight which is in effect a light meter that produces an audio tone that rises in pitch the more light it detects. The targets used are white in the centre, shading to black at the edge and are illuminated by a small spot light. We have recently obtained the manufacturer's specification for the acoustic sight:

Specifications:

Lens diameter 32 mm

Lens focal length 133 mm

Tube diameter 26 mm

Distance setting (fixed) 10 m

Receiver surface at 10m distance 36 mm dia.

Active photodiode area 0.7 mm dia.

input Voltage 7-12 V DC

supply Current 100 mA

output frequency
(Fundamental frequency from the outside with a screwdriver adjustable)
a) General
b) With 14 000 Lx white surface illuminated at 10m distance

0 to 20 kHz
2.4 kHz + - 5%

Point correction 10m distance 0.7 mm per detent (click of sight movement)

Weight without mounting 0.38 kg

power Supply

output voltage 9 V DC

Maximum output current 200 mA

connection 2.1 mm dia., NF connector

Headphones with volume control impedance 8 Ohm

Can anyone explain or point me in the direction of the metering principles that are likely to be in use with this device? We are not trying to reverse engineer it, but trying to better understand it in order to ensure we are getting optimum performance.

BTW the shooter in the video has a personal best score of 596 ex 600.

Thanks exmpa

 

[artyman]

The device obviously measures reflected light from the source, presumably fairly well focused from an input point of view, you wouldn't want it picking up spurious signals from around the actual target. Based on the signal from the reflectance a tone is produced which varies based on reflectance input levels.

 

[exmpa]

The device obviously measures reflected light from the source, presumably fairly well focused from an input point of view, you wouldn't want it picking up spurious signals from around the actual target. Based on the signal from the reflectance a tone is produced which varies based on reflectance input levels.

We have a fairly good idea of the based upon observation and evidence of side bar returns (change in pitch due to edge illumination). The interesting thing is that if the 10 ring inwards is removed; cut out or shot away; the sight response seems only slightly degraded. The Inner 10 ring pitch is not attainable, but the 10 ring pitch is identifiable.

I can provide dimensions and shading areas of the targets if anyone is interested.

exmpa